If you want easy access to your tunnels you can make the upper layers removable. After you have the bottom 4 inches in place, set the next layer on top and roughly cut it in place. get some #2 pencils, break them off about 4 inches and sharpen them on both sides, fit them into the bottom layers, and when you place the upper layers on top, the pencil points will serve as reference points for creating the matching holes in the upper layers. Once the upper layers fit snuggly, you can finish detailing the scenery. You can hide the seam in a lot of different ways depending on what your terrain is. This also makes the layout more portable and easier to store and transport if the height is a problem. MR magazine had a similar solution to a problem, but I think they used nails with a plywood base and foam mountains. You may want your copper mine to be removable with pegs to keep it in the right location also.